Several herb medicines including (a) Astragali radix, (b) Cnidii rhizoma, (c) Angelicae radix, and (d) Hoelen have been reported to exhibit stimulating activities on the immune system and are relatively nontoxic. Polysaccharides were isolated from these plant medicines. Their physicochemical properties were characterized including sugar contents and molecular weights. Effects on production of interleukin (IL)-2, IL-4 and IL-6 were studied in spleen cells of mice treated with Angelica PS or Hoelen PS followed by immunization with pneumococcal type 9V PS-ply conjugate. Mice treated with Angelica PS or Hoelen PS and type 9V PS-ply conjugate produced 1.5 to 3.9 fold higher IL-2 and 4.6 to 6.4 fold higher IL-4 levels than the group given the conjugate alone. Plant PS did not stimulate IL-6 production. Thus, these plant PS enhanced the pneumococcal PS conjugate to produce more IL-2 and IL-4. The antibody response to the type 9V PS-ply conjugate was studied in adult mice treated with Angelica PS or Hoelen PS. Serum levels of anti-9V PS IgG and IgM antibodies were enhanced in the mice by Angelica PS or Hoelen PS compared to non-treated controls. These results indicate that the enhanced immune response to pneumococcal PS-protein conjugate induced by plant PS may be mediated through the activities of cytokines.